Trunked communication systems are known to comprise a plurality of communication units, a limited number of communication resources that are transceived via a predetermined number of repeaters, or base stations, and a communication resource allocator that allocates a limited number of communication resources among the plurality of communication units. The communication resources may comprise a TDM bus, a carrier frequency, a pair of carrier frequencies, or any RF transmission means. Of the communication resources, one is selected to act as a control channel which transceives operational data between the communication resource allocator and the communication units. The communication units, which may be portable radios and/or mobile radios, are arranged into talk groups by commonality of use. For example, a talk group may comprise communications that are operated by a police department while another talk group comprises communication units operated by a fire department.
In addition to basic communications between talk group members, trunking communication systems offer a variety of services that a particular communication unit can subscribe to. For example, a communication unit may subscribe to such services as telephone interconnections, facsimile transmissions, private calling, wide area calling and the like. The communication resource allocator maintains a data base of the systems services that a communication unit has subscribed to. However, the communication unit does not contain such information. Thus, when a communication unit desires to perform a particular function it transmits in a request via a control channel, and once received by the communication resource allocator, the communication resource allocator determines whether that particular communication unit can perform such a service. If the communication unit has not subscribed to that service, the communication resource allocator transmits a denial message to that particular communication unit. This prevents the communication unit from accessing the selected service but does not prevent the communication unit from transmitting the same request at a subsequent time and having to receive the denial message again.
With the communication unit transmitting service requests that will not be granted over and over again, the control channel becomes unnecessarily busy. Therefore, a need exists for a method that will allow a communication unit to maintain a data base of the system services that it can perform such that control channel traffic is reduced.